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The Great Barrier Reef is the largest coral reef system in the world, composed of over 2,900 individual reefs and 900 islands stretching for 2,600 kilometers over an area of approximately 344,400 square kilometers. The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia.

The Great Barrier Reef can be seen from outer space and is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of tiny organisms, known as coral polyps. The Great Barrier Reef supports a wide diversity of life, and was selected as a World Heritage Site in 1981. CNN has labeled it one of the 7 Natural Wonders of the World. The Queensland National Trust has named it a State Icon of Queensland.

The history of the development of the Great Barrier Reef is complex; after Queensland drifted into tropical waters, the history is largely influenced by how reefs fluctuate as the sea level changes. They can increase in diameter from 1 to 3 centimeters per year, and grow vertically anywhere from 1 to 25 centimeters per year; however, they are limited to growing above a depth of 150 meters due to their need for sunlight, and cannot grow above sea level. The land that formed the substrate of the current Great Barrier Reef was a coastal plain formed from the eroded sediments of the Great Dividing Range with some larger hills.

From 20,000 years ago until 6,000 years ago, the sea level rose steadily. As it rose, the corals could then grow higher on the hills of the coastal plain. By around 13,000 years ago the sea level was 60 meters lower than the present day, and corals began to grow around the hills of the coastal plain, which were, by then, continental islands. As the sea level rose further still, most of the continental islands were submerged. The corals could then overgrow the hills, to form the present cays and reefs. Sea level on the Great Barrier Reef has not risen significantly in the last 6,000 years. The CRC Reef Research Center estimates the age of the present, living reef structure at 6,000 to 8,000 years old.

The Great Barrier Reef supports a diversity of life, including many vulnerable or endangered species, some of which may be endemic to the reef system. Thirty species of whales, dolphins, and porpoises have been recorded in the Great Barrier Reef, including the dwarf minke whale, Indo-Pacific humpback dolphin, and the humpback whale. Large populations of dugongs live there.

Six species of sea turtles come to the reef to breed. The green sea turtles on the Great Barrier Reef have two genetically distinct populations. Fifteen species of seagrass in beds attract the dugongs and turtles,and provide a habitat for fish.

Salt water crocodiles live in mangrove and saltmarshes on the coast near the reef. Around 125 species of shark, stingray, skates or chimera live on the reef. Close to 5,000 species of mollusc have been recorded on the reef, including the giant clam and various nudibranchs and cone snails. At least seven species of frog can be found on the islands.

The most significant threat to the Great Barrier Reef is climate change. Mass coral bleaching events due to rising ocean temperatures occurred in of the summers of 1998, 2002 and 2006, and coral bleaching will likely become an annual occurrence.

Climate change has implications for other forms of life on the Great Barrier Reef as well - some fish's preferred temperature range lead them to seek new areas to live, thus causing chick mortality in seabirds that prey on the fish. Climate change will also affect the population and available habitat of sea turtles.

As the largest commercial activity in the region, it was estimated in 2003 that tourism in the Great Barrier Reef generates over AU$4 billion annually. Approximately two million people visit the Great Barrier Reef each year.

A variety of boat tours and cruises are offered, from single day trips, to longer voyages. Boat sizes range from dinghies to super-yachts. Glass-bottomed boats and underwater observatories are also popular, as are helicopter flights. By far, the most popular tourist activities on the Great Barrier Reef are snorkeling and diving, for which pontoons are often used, and the area is often enclosed by nets. The outer part of the Great Barrier Reef is favored for such activities, due to water quality.